Typical ion beam implanters include an ion source for generating positively charged ions from source materials. The generated ions are formed into a beam and directed along a predetermined beam path to an implantation station. The ion beam implanter may include beam forming and shaping structures extending between the ion source and the implantation station. The beam forming and shaping structures maintain the ion beam and bound an elongated interior cavity or passageway through which the beam passes en route to the implantation station. When operating an implanter, this passageway can be evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with residual gas molecules.
The mass of an ion relative to the charge thereon (e.g., charge-to-mass ratio) affects the degree to which it is accelerated both axially and transversely by an electrostatic or magnetic field. Therefore, the beam which reaches a desired area of a semiconductor wafer or other target can be made very pure since ions of undesirable molecular weight can be deflected to positions away from the beam and implantation of other than desired materials can be avoided. The process of selectively separating ions of desired and undesired charge-to-mass ratios is known as mass analysis. Mass analyzers typically employ a mass analysis magnet creating a dipole magnetic field to deflect various ions in an ion beam via magnetic deflection in an arcuate passageway which will effectively separate ions of different charge-to-mass ratios.
For shallow depth ion implantation, high current, low energy ion beams are desirable. In this case, the reduced energies of the ions cause some difficulties in maintaining convergence of the ion beam due to the mutual repulsion of ions bearing a like charge. High current ion beams typically include a high concentration of similarly charged ions which tend to diverge due to mutual repulsion.
To maintain low energy, high current ion beam integrity at low pressures, a plasma may be created to surround the ion beam. Ion beams typically propagate through a weak plasma that is a byproduct of the beam interactions with the residual or background gas. This plasma tends to neutralize the space charge of the ion beam, thereby largely eliminating transverse electric fields that would otherwise disperse the beam.
In ion implantation systems, there remains a need for a beam containment apparatus and methodologies for use with high current, low energy ion beams which may be operated at low pressures and provide uniform beam containment along the length of a mass analyzer beam guide.